As the volume of Internet traffic grows, providers of web content and applications increasingly need to deliver content from multiple servers at widely-separated locations in order to sustain a good end-user experience under high traffic loads. This need generates several difficult challenges, including, among others:                how to guarantee the fault-tolerance of such a multiple-server system in the face of failure of one or more individual servers;        how to control the way in which requests from end-users are distributed to each server according to important content provider policy constraints imposed for economic, contractual or other reasons; and        how to guarantee high performance as experienced by end-users as network conditions change.        
This invention solves these and other problems by providing a means to distribute network (e.g., Internet) traffic according to a configurable set of rules. The rules can be configured to take into account key factors such as:                server availability.        specific requirements of content providers who deploy the invention, e.g., distribution based upon geography, position in IP address space, load share, etc.        state of the network (Internet) at any given moment, including measures of network latency.        
These rules together provide an extremely fine-grained level of network Internet traffic control to providers of Internet content and applications, enabling them to dramatically improve the end-user experience (measured by speed of request resolution, associated download time, and the availability of servers) over that provided by conventional web servers and mirrored server farms.
There are many potential uses for the invention. One use is to provide a stand-alone service directing traffic exclusively to a set of designated servers managed by a single organization. The invention may also be used in more general ways—for example, one or more of the designated destinations can refer to servers (or server collections) outside the organization's control. The latter case includes, for example, Content Delivery Networks (CDNs), as well as local load-balancing servers, as potential destinations. The invention can also be used, e.g., to provide the DNS (Domain Name Service) component of a Content Delivery Network itself. It can be deployed as a service on behalf of subscribers, or it can be deployed as software to be used directly by subscribers themselves.